Edithvale-Seaford Wetlands. Credit: illarionovdv / iStock / Getty Images Plus.
The Edithvale-Seaford Wetlands – also known as the Seaford Swamp – is a significant natural habitat within the Melbourne metropolitan area just 35km southeast of the city centre. It is a prime location for watching southeastern Australia’s wetland birds and other wildlife.
But for decades the conservation of the swamp may have been misguided.
Findings from historical sediment analysis published in CSIRO Publishing’s journal Marine and Freshwater Research reveals the swamp wasn’t always a freshwater system as it is today.
Seaford Swamp sits on the ancestral lands of the Indigenous Bunurong People. Today it is a fresh marsh and a “back of dune wetland” – a kind of wetland which forms behind coastal sand dunes. The dominant plant is the common reed (Phragmites australis) which grows around the world.
A team of palaeoecologists and archaeologists took core samples up to 2m deep in 2022.
The cores were analysed for traces of diatoms (single-celled algae) and pollen. Carbon and lead isotopes were used to date the samples. The 2m-deep core was estimated to trace the evolution of the habitat over nearly 7,000 years.
The upper 50cm of the longest core, taken from the north of the swamp, showed a sudden change in the ecosystem in the mid-to-late 19th century. Other shorter cores also indicated this shift.
Seaford Swamp was once salty due to saltwater inflow from the sea. Agriculture and urban development following European settlement blocked much of this flow.
“The longest core showed an abrupt change from marine–brackish to saline and ultimately the present fresh condition,” the authors write. “Saltmarsh declined and was replaced by reed, sedge and rush species. Cores from the centre and south of the wetland showed a similar history, albeit more variable in the south, yet still reflecting a sustained change from marine–estuarine conditions to fresh–brackish.”
This abrupt change, the authors say, “appeared to correspond with hydrological engineering of the site from 1870 for agricultural and urban development. Increased catchment runoff, including from urban areas, are likely to have sustained the freshwater state”.
The researchers say basing conservation programs on human memory of how environments used to be alone “can be punctuated, subjective and dominated by recent experience”.
Historical analysis of sediments, however, “can provide evidence of state and transition beyond memory and documentary records”.
Edithvale-Seaford Wetlands was listed in 2001 as an ecologically important wetland under the internationally-recognised Ramsar Convention which aims to prevent wetland loss. However, the authors note that it was listed as a “freshwater system” and that conservation efforts should focus on maintaining this state.
But the authors say their findings present new options as to which state of the Seaford Swamp should be conserved – its current freshwater habitat, or the earlier salty estuarine one.
“The extended record of change in the Seaford Swamp identified in this investigation ‘opens up’ the suite of target states that management may consider. This could include the present freshwater state or a more saline one that might reduce the proliferation of reeds and reflect that witnessed by Bunurong People in the past,” they say.
The authors also suggest that future sea level rise due to human-induced climate change may raise alternative options for how the wetlands are managed.
They conclude: “Whichever, it remains that management needs to also consider the listing criteria that underpin its original nomination as well as the obligation to maintain the ecological character, which includes the key features of the biodiversity, the ecological processes that sustain the wetlands, and the ecosystem services that it provides for people.”